Degradable fiber systems for stimulation

ABSTRACT

A method for minimizing the amount of metal crosslinked viscosifier necessary for treating a wellbore with proppant or gravel is given. The method includes using fibers to aid in transporting, suspending and placing proppant or gravel in viscous carrier fluids otherwise having insufficient viscosity to prevent particulate settling. Fibers are given that have properties optimized for proppant transport but degrade after the treatment into degradation products that do not precipitate in the presence of ions in the water such as calcium and magnesium. Crosslinked polymer carrier fluids are identified that are not damaged by contaminants present in the fibers or by degradation products released by premature degradation of the fibers.

This application is a continuation application of U.S. patentapplication Ser. No. 12/946,518, filed Nov. 15, 2010, which is acontinuation application of U.S. patent application Ser. No. 12/130,130,filed May 30, 2008, now issued as U.S. Pat. No. 7,833,950, and is adivisional application of U.S. patent application Ser. No. 11/846,358,filed Aug. 28, 2007, now U.S. Pat. No. 7,380,601 which is a divisionalof U.S. patent application Ser. No. 11/156,966, filed Jun. 20, 2005, nowU.S. Pat. No. 7,275,596 and claims the benefit of the filing datesthereof. These four patent applications and patents are hereinincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The invention relates to particulate transport; more particularly itrelates to particulate transport in oilfield stimulation treatments;even more particularly it relates to proppant and gravel transportslurried in viscous carrier fluids having insufficient viscosity toprevent particulate settling; even more particularly it relates to theinclusion in such fluids of fibers that aid in transport and inhibitsettling; most particularly it relates to such fibers that degrade afterthe treatment into degradation products that do not precipitate in thepresence of ions in the water such as calcium and magnesium, and to theselection of crosslinked polymer carrier fluids that are not damaged bycontaminants present in the fibers or degradation products released bypremature degradation of the fibers.

Hydrocarbons (oil, condensate, and gas) are typically produced fromwells that are drilled into the formations containing them. Often, for avariety of reasons, such as inherently low permeability of theresource-containing formation or damage to the formation caused bydrilling and completion of the well, the flow of hydrocarbons into thewell and thus to the surface is undesirably low. When that is the case,the well is often “stimulated”. One of the most common forms ofstimulation is hydraulic fracturing, in which a fluid is injected intothe well and then into the formation at a pressure above the “fracture”pressure of the formation. A fracture is formed and grows into theformation, greatly increasing the surface area through which fluids mayflow into the well. When the injection pressure is released, thefracture closes; consequently, a particulate material, called a“proppant” is included in the fracturing fluid so that when the pressureis released, the fracture cannot close completely, but rather closes onthe proppant and the fracture faces are thus held apart by a bed ofproppant through which the fluids may then flow to the well. Thefracturing fluid normally must have a minimal viscosity that serves twopurposes. First, the more viscous the fluid the more readily thefracture will be widened by injection of more fluid, and, second, a moreviscous fluid will more readily transport proppant, hence the term“carrier” fluid. However, when the fluid is viscosified with a polymer,as is often the case, especially with a crosslinked polymer, at leastsome of the polymer or crosslinked polymer is left in the fracture afterthe treatment. This viscosifier left in the fracture inhibits the flowof desirable fluids out of the formation, through the fracture, into thewellbore, and to the surface for recovery. To some extent the need forviscosity can be offset by injecting fluid at faster rates, but for avariety of reasons, such as the limitations of the equipment and thecosts, this is not always a desirable procedure. The viscosity may alsobe provided by non-polymeric methods, such as the use of foams,emulsions, and viscoelastic surfactant fluid systems, but sometimesthese may not be the solution of choice. Operators may also choose touse the least damaging polymers available, but these may be expensive.

One solution some operators have chosen to minimize cost and polymerdamage is to use as little polymer as possible. One such method isslickwater (also called waterfrac) treatments (with minimal proppant anda fluid viscosity, for example, of only about 3 cP, as opposed toconventional jobs with crosslinked polymer carrier fluids that typicallyhave viscosities of at least 100 cP, and usually much more. To make upfor the low viscosity, such jobs are usually pumped at high rates tohelp create the fracture and to transport the proppant, but fractureheight growth may be excessive, very little proppant is placed, and theproppant may settle into the bottom of the fracture. This settling canoccur as the fluid breaks or simply as a result of inadequate initialdesigned viscosity. Sometimes operators try to compromise with acombination of hybrid of a slickwater and a conventional job, which mayresult in the disadvantages of each.

Typically, when operators choose to use a more conventional fracturingmethod with polymer-based carrier fluids, they try to use the lowestpossible polymer concentration, to minimize the damage caused by thepolymer. It has recently been found that fibers included in the slurryof proppant in carrier fluid may serve to aid in the transport ofproppant at lower viscosities and/or lower slurry flow rates (see SPE68854 and SPE 91434) provided that fibers of the appropriate length,diameter, and stiffness are chosen and used in the right concentration.Such fibers also have the advantages that they improve the properties ofthe proppant pack, such as its fluid conductivity, its ability to aid insand control, and resistance to flowing back of proppant particles intothe wellbore. However, although the treatments have been verysuccessful, there is still room for improvement; the materialspreviously typically used for the fibers either (glass or novoloid) didnot degrade under formation conditions or did not have optimizedstiffness for proppant transport or (polyethylene terephthalate)degraded into products that could reduce the final effectiveness of thefracture.

SUMMARY OF THE INVENTION

A method of treating a wellbore and a formation penetrated by thewellbore is given in which the method contains the step of injecting aslurry of fibers and proppant in a viscous carrier fluid. The viscosityof the carrier fluid in the absence of fibers is insufficient to preventproppant settling during transport. The fibers degrade after thetreatment into products that do not precipitate in the presence ofcalcium or magnesium ions. The step of injecting may optionally becarried out above the fracture pressure of the formation, in which casethe viscosity of the carrier fluid in the absence of fibers mayoptionally be insufficient to prevent proppant settling in the fracturebefore closure of the fracture after injection is stopped.

Suitable fibers are selected from substituted and unsubstituted lactide,glycolide, polylactic acid, polyglycolic acid, copolymers of polylacticacid and polyglycolic acid, copolymers of glycolic acid with otherhydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containingmoieties, and copolymers of lactic acid with other hydroxy-, carboxylicacid-, or hydroxycarboxylic acid-containing moieties, and mixtures ofthose materials

Typically, the fibers have a length of about 2 to about 25 mm,preferably about 3 to about 18 mm. Typically, the fibers have a denierof about 0.1 to about 20, preferably about 0.15 to about 6. The fibersdegrade at formation temperature in a time between about 4 hours and 100days.

In one embodiment, the carrier fluid contains a borate crosslinkedpolymer. The polymer may be guar at a concentration of between about1.92 g/L (about 16 ppt) and about 3.6 g/L (about 30 ppt), preferablybetween about 1.92 g/L (about 16 ppt) and about 2.88 g/L (about 24 ppt).The carrier fluid may contain a pH control agent present in an amountsufficient to neutralize any acid present in the fibers before theinjection and to neutralize any acid generated by the fibers before thefracture closes. The acid is neutralized for example to an extentsufficient to maintain the viscosity of the fluid in the presence offibers above about 75 cP at a shear rate of 100 sec⁻¹ at the temperatureof the formation. The pH control agent is selected from amines oralkaline earth, ammonium, and alkali metal salts of sesquicarbonates,carbonates, hydroxides, oxides, bicarbonates, or organic carboxylates,for example the amines triethanolamine or tetraethylenepentamine. Theborate crosslinker is boric acid or sodium tetraborate decahydrate. Thefluid optionally also contains a crosslinking delay agent, for examplesorbitol.

In another embodiment, the carrier fluid contains a polymer, for examplecarboxymethylhydroxypropyl guar, crosslinked with a zirconate. Thepolymer concentration is between about 2.64 g/L (about 22 ppt) to about3.6 g/L (about 30 ppt). The zirconate crosslinker is for examplezirconium lactate. The fluid optionally contains a gel stabilizer, forexample sodium thiosulfate.

In yet another embodiment, the fluid may contain a breaker.

In yet further embodiments, the carrier fluid may be thickened with aviscoelastic surfactant, for example a zwitterionic surfactant, forexample a betaine, or an amidoamine oxide. In yet further embodiments,the carrier fluid may be an emulsion, may be foamed or energized, or maybe a solution of diutan, xanthan, scleroglucan, orhydroxyethylcellulose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the rate of decomposition of a fiber used in the inventionas a function of temperature.

FIG. 2 presents the viscosities of a number of fluids, some suitable forthe invention and some not, as a function of time at severaltemperatures.

DETAILED DESCRIPTION OF THE INVENTION

We have found that suitable fibers can assist in transporting,suspending and placing proppant in hydraulic fracturing and gravelpacking and then degrade to minimize or eliminate the presence of fibersin the proppant pack without releasing degradation products that eithera) react with certain multivalent ions present in the fracture water orgravel packing carrier fluid, or formation water to produce materialsthat hinder fluid flow, or b) decrease the ability of otherwise suitablemetal-crosslinked polymers to viscosify the carrier fluid. We will referto a system in which suitable fibers and a fluid viscosified with asuitable metal-crosslinked polymer system are used to slurry andtransport proppant as a “fiber/polymeric viscosifier” system or an “FPV”system.

The FPV system will be described primarily in terms of hydraulicfracturing, but it is also suitable for gravel packing, or forfracturing and gravel packing in one operation (called, for example fracand pack, frac-n-pack, frac-pack, StimPac treatments, or other names),which are also used extensively to stimulate the production ofhydrocarbons, water and other fluids from subterranean formations. Theseoperations involve pumping a slurry of “proppant” (natural or syntheticmaterials that prop open a fracture after it is created) in hydraulicfracturing or “gravel” in gravel packing. In low permeabilityformations, the goal of hydraulic fracturing is generally to form long,high surface area fractures, that greatly increase the magnitude of thepathway of fluid flow from the formation to the wellbore. In highpermeability formations, the goal of a hydraulic fracturing treatment istypically to create a short, wide, highly conductive fracture, in orderto bypass near-wellbore damage done in drilling and/or completion, toensure good fluid communication between the rock and the wellbore andalso to increase the surface area available for fluids to flow into thewellbore. Gravel is also a natural or synthetic material, which may beidentical to, or different from, proppant. Gravel packing is used for“sand” control. Sand is the name given to any particulate material, suchas clays, from the formation that could be carried into productionequipment. Gravel packing is a sand-control method used to preventproduction of formation sand, in which, for example a steel screen isplaced in the wellbore and the surrounding annulus is packed withprepared gravel of a specific size designed to prevent the passage offormation sand that could foul subterranean or surface equipment andreduce flows. The primary objective of gravel packing is to stabilizethe formation while causing minimal impairment to well productivity.Sometimes gravel packing is done without a screen. High permeabilityformations are frequently poorly consolidated, so that sand control isneeded; they may also be damaged, so that fracturing is also needed.Therefore, hydraulic fracturing treatments in which short, widefractures are wanted are often combined in a single continuous (“fracand pack”) operation with gravel packing. For simplicity, in thefollowing we may refer to any one of hydraulic fracturing, fracturingand gravel packing in one operation (frac and pack), or gravel packing,and mean them all.

The FPV system is particularly suitable for fracturing tight gas wells,which are typically low-permeability environments with extended fractureclosure times; in such cases the fracture may remain open for hoursafter injection ceases, and the carrier fluid may break and no longersuspend the proppant. The FPV system allows lower polymer loadings,reduced fracture height growth (because of the lower viscosity that canbe used), reduced proppant settling, and increased retained permeability(improved dimensionless fracture conductivity), all of which result inimproved production rates. The FPV system is also particularly suitablefor gravel packing when dense brines are used that contain highconcentrations of calcium or other ions that would precipitate with thedegradation products of other degradable fibers (for example up to12,000 ppm calcium). It is also particularly suitable for situations inwhich the connate water, that will flow into the fracture after thetreatment, is high in such ions as calcium and magnesium.

Some fibers previously used for transporting, suspending, and placingproppant, such as polyethylene terephthalate, degrade into by-productsthat may precipitate salts in the presence of excessive amounts ofcalcium or magnesium in the connate water. Preventive measures may betaken with other fibers, such as, but not limited to, pumping a pre-padand/or pumping an acid or a chelating dissolver, adsorbing or absorbingan appropriate chelating agent onto or into the fiber, or incorporatingin the fluid precipitation inhibitors or metal scavenger ions thatprevent precipitation. With the fibers of the FPV system screening forthese cations in the connate water and applying these preventive orcorrection measures are not necessary.

Early treatments using fibers to help transport proppant, sometimescalled “fiber assisted transport” treatments were typically slickwater(also called waterfrac) treatments (with minimal proppant and a fluidviscosity, for example, of only about 3 cP), as opposed to conventionaltreatments with crosslinked polymer carrier fluids that typically haveviscosities of at least 100 cP, and usually much more. It has now beenfound that the treatments with fibers in the fluid are improved iflow-concentration crosslinked polymer carrier fluids are used, forexample having a viscosity of at least about 50 cP, preferably at leastabout 75 cP, (at 100 sec⁻¹) at the temperature at which the fluid isused, especially in stiffer rocks commonly found in tight gasreservoirs, in which the higher viscosity provides increased fracturewidth. The FPV system de-couples proppant transport characteristics ofthe fluid from the fluid viscosity. It allows a much lower polymerloading to be used to achieve proppant placement without sacrificingproppant coverage; this means less chance of undesired fracture heightgrowth and reduced fracture damage due to polymer or crosslinkedpolymer. The viscosity needed depends upon factors such as the stiffnessof the rock; the amount, identity, size and stiffness of the fibers; thepumping rate and duration; and only to some extent the proppant size,concentration and density. The viscosity needed can be determined bymathematical modeling or by experiments, such as those known as slotflow experiments, known in the industry. Oilfield service companies andcontract testing companies can make such determinations.

Suitable materials for the fibers of the invention include substitutedand unsubstituted lactide, glycolide, polylactic acid, polyglycolicacid, a copolymer of polylactic acid and polyglycolic acid, a copolymerof glycolic acid with other hydroxy-, carboxylic acid-, orhydroxycarboxylic acid-containing moieties, a copolymer of lactic acidwith other hydroxy-, carboxylic acid or hydroxycarboxylicacid-containing moieties, or mixtures of the preceding. Other materialssuitable for use are all those polymers of hydroxyacetic acid (glycolicacid) with itself or other hydroxy-, carboxylic acid-, orhydroxycarboxylic acid-containing moieties described in U.S. Pat. Nos.4,848,467; 4,957,165; and 4,986,355, all three hereby incorporated byreference. Suitable materials for the fibers of the invention are alsodescribed in U.S. Patent Application Publication Nos. 2003/002195 and2004/0152601, both of which are hereby incorporated by reference and areassigned to the assignee of the present application. Other polymers, forexample those that degrade at other temperatures, or other pH's, orthose that have different chemical compatibilities, may be used, forexample polyvinyl alcohol, optionally with suitable carrier fluidadjustment.

Excellent materials for the fibers of the invention are solid cyclicdimers, or solid polymers, of certain organic acids, that hydrolyzeunder known and controllable conditions of temperature, time and pH; thedegradation products are organic acids. One example of a suitablematerial is the solid cyclic dimer of lactic acid (known as “lactide”),which has a melting point of 95 to 125° C., (depending upon the opticalactivity). Another is a polymer of lactic acid, (sometimes called apolylactic acid (or “PLA”), or a polylactate, or a polylactide). Anotherexample is the solid cyclic dimer of gylycolic acid (known as“glycolide”), which has a melting point of about 86° C. Yet anotherexample is a polymer of glycolic acid (hydroxyacetic acid), also knownas polyglycolic acid (“PGA”), or polyglycolide. Another example is acopolymer of lactic acid and glycolic acid. These polymers andcopolymers are polyesters. Generally the cyclic dimmers are polymerizedto form the final polymer from which the fiber is made, but for lowtemperature operations fiber may be made directly from the solid cyclicdimmers. The as-received commercially available materials may containsome free acid, for example up to about 5%) and some solvent, typicallywater.

NatureWorks LLC, Minnetonka, Minn., USA, owned by Cargill Inc.,Minneapolis, Minn., USA, produces the solid cyclic lactic acid dimercalled “lactide” and from it produces lactic acid polymers, orpolylactates, with varying molecular weights and degrees ofcrystallinity, under the generic trade name NatureWorks™ PLA. The PLA'scurrently available from NatureWorks most commonly have molecularweights of up to about 100,000, although any polylactide (made by anyprocess by any manufacturer) and any molecular weight material of anydegree of crystallinity may be used in the embodiments of the Invention.The PLA polymers are solids at room temperature and are hydrolyzed bywater to form lactic acid. Those available from NatureWorks typicallyhave crystalline melt temperatures of from about 120 to about 170° C.,but others are obtainable. Poly(d,l-lactide) is available fromBio-Invigor, Beijing and Taiwan, with molecular weights of up to500,000. Bio-Invigor also supplies polyglycolic acid (also known aspolyglycolide) and various copolymers of lactic acid and glycolic acid,often called “polyglactin” or poly(lactide-co-glycolide). The rates ofthe hydrolysis reactions of all these materials are governed, amongother factors, by the molecular weight, the crystallinity (the ratio ofcrystalline to amorphous material), the physical form (size and shape ofthe solid), and in the case of polylactide, the amounts of the twooptical isomers. (The naturally occurring l-lactide forms partiallycrystalline polymers; synthetic dl-lactide forms amorphous polymers.)Amorphous regions are more susceptible to hydrolysis than crystallineregions. Lower molecular weight, less crystallinity and greatersurface-to-mass ratio all result in faster hydrolysis. Hydrolysis isaccelerated by increasing the temperature, by adding acid or base, or byadding a material that reacts with the hydrolysis product(s).

Homopolymers can be more crystalline; copolymers tend to be amorphousunless they are block copolymers. The extent of the crystallinity can becontrolled by the manufacturing method for homopolymers and by themanufacturing method and the ratio and distribution of lactide andglycolide for the copolymers. Polyglycolide can be made in a porousform. Some of the fibers dissolve very slowly in water before theyhydrolyze

The fibers of the invention may be coated to slow the hydrolysis.Suitable coatings include polycaprolate (a copolymer of glycolide andepsilon-caprolactone), and calcium stearate, both of which arehydrophobic. Polycaprolate itself slowly hydrolyzes. Generating ahydrophobic layer on the surface of the materials for the fibers of theinvention by any means delays the hydrolysis. Note that coating here mayrefer to encapsulation or simply to changing the surface by chemicalreaction or by forming or adding a thin film of another material, forexample an oil. The degradation does not occur until water contacts thematerials the fibers.

The fibers self-destruct in situ, that is, in the location where theyare placed. Although normally that is in a proppant pack in a fracture,that location may also be part of a suspension in the wellbore, inperforations, in a gravel pack, as a component of a filter cake on thewalls of a wellbore or of a fracture, or in natural fractures or vugs ina formation. The fiber/polymeric viscosifier system may be used incarbonates and sandstones. A particular advantage of these materials isthat the fibers of the invention and the generated acids are non-toxicand are biodegradable.

Suitable fibers have a length of about 2-25 mm, preferably about 3-18mm, most preferably about 6 mm; they have a denier of about 0.1-20,preferably about 0.15-6, most preferably about 1.4 Such fibers areoptimized for particle transport.

Suitable polymers for making the metal-crosslinked polymer viscosifiersinclude, for example, polysaccharides such as substitutedgalactomannans, such as guar gums, high-molecular weight polysaccharidescomposed of mannose and galactose sugars, or guar derivatives such ashydroxypropyl guar (HPG), carboxymethylhydroxypropyl guar (CMHPG) andcarboxymethyl guar (CMG), hydrophobically modified guars,guar-containing compounds, and synthetic polymers. Crosslinking agentsbased on boron, titanium, zirconium or aluminum complexes are typicallyused to increase the effective molecular weight of the polymer and makethem better suited for use in high-temperature wells.

Other suitable classes of effective water-soluble polymers (providedthat specific examples chosen are compatible with the fibers of theinvention) include polyvinyl polymers, polymethacrylamides, celluloseethers, lignosulfonates, and ammonium, alkali metal, and alkaline earthsalts thereof. More specific examples of other typical water solublepolymers are acrylic acid-acrylamide copolymers, acrylicacid-methacrylamide copolymers, polyacrylamides, partially hydrolyzedpolyacrylamides, partially hydrolyzed polymethacrylamides, polyvinylalcohol, polyvinyl acetate, polyalkyleneoxides, carboxycelluloses,carboxyalkylhydroxyethyl celluloses, hydroxyethylcellulose, othergalactomannans, heteropolysaccharides obtained by the fermentation ofstarch-derived sugar (e.g., xanthan gum), and ammonium and alkali metalsalts thereof.

Cellulose derivatives are used to a smaller extent, such ashydroxyethylcellulose (HEC) or hydroxypropylcellulose (HPC),carboxymethylhydroxyethylcellulose (CMHEC) and carboxymethycellulose(CMC), with or without crosslinkers. Xanthan, diutan, and scleroglucan,three biopolymers, have been shown to have excellent proppant-suspensionability even though they are more expensive than guar derivatives andtherefore have been used less frequently unless they can be used atlower concentrations.

Linear (not cross-linked) polymer systems may be used, but generallywill not be preferred because they will require more polymer. Allcrosslinked polymer systems may be used, including for example delayed,optimized for high temperature, optimized for use with sea water,buffered at various pH's, and optimized for low temperature. Anycrosslinker may be used, for example boron, titanium, and zirconium.Suitable boron crosslinked polymers systems include by non-limitingexample, guar and substituted guars crosslinked with boric acid, sodiumtetraborate, and encapsulated borates; borate crosslinkers may be usedwith buffers and pH control agents such as sodium hydroxide, magnesiumoxide, sodium sesquicarbonate, and sodium carbonate, amines (such ashydroxyalkyl amines, anilines, pyridines, pyrimidines, quinolines, andpyrrolidines, and carboxylates such as acetates and oxalates) and withdelay agents such as sorbitol, aldehydes, and sodium gluconate. Suitablezirconium crosslinked polymer systems include by non-limiting example,those crosslinked by zirconium lactates (for example sodium zirconiumlactate), triethanolamines, 2,2′-iminodiethanol, and with mixtures ofthese ligands, including when adjusted with bicarbonate. Suitabletitanates include by non-limiting example, lactates andtriethanolamines, and mixtures, for example delayed with hydroxyaceticacid. Any other chemical additives can be used or included provided thatthey are tested for compatibility with the fibers and fiber degradationproducts of the invention (neither the fibers or their degradationproducts or the chemicals in the fluids interfere with the efficacy ofone another or with fluids that might be encountered during the job,like connate water or flushes). For example, some of the standardcrosslinkers or polymers as concentrates usually contain materials suchas isopropanol, n-propanol, methanol or diesel oil.

Because the fibers degrade to release acid, two metal crosslinkedpolymer systems are particularly preferred. These preferred fluids arerelatively insensitive to the release of acid that accompanies thedegradation of the fibers. (Other fluids may be used, especially undermild conditions or for short treatments, but may be less stable.) Thepreferred borate crosslinked fluids initially have high a pH, forexample above about 11.5. Degradation of 3.6 g/L (30 ppt) of fibers willdecrease the pH to about 9 to 9.5. At that pH borate fluids are stillcrosslinked. If the pH drops below about 9, however, the fluid starts tobreak. Conversely, the rate of degradation of PLA is lowest at about apH of 5; it increases at lower and higher pH's, increasing faster athigher pH's than at low. The fiber described in Example 1 below has anexpected downhole life of about 5 to 6 hours at a pH of 6 to 7 at 121°C. 250° F.; about 3 to 4 hours at a pH of about 10 at that temperature;and about 2 to 3 hours at a pH of about 12 at that temperature. At a pHof about 12, the fiber has an expected downhole life of about 5 to 6hours at 104° C. 220° F. and of about 15 to 18 hours at about 93° C.200° F. Preferably, the fibers degrade in a time at formationtemperature of from about 4 hours to about 100 days. Triethanolaminestabilizes the fluids to the acid released from the fibers up to aconcentration of about 2 gpt of an 85% triethanolamine solution inwater. Another reason why these fluids are preferred is that it isbetter to use delayed fluids with fibers, because fiber dispersion inwater is better before crosslinking.

The first preferred metal-crosslinked polymer system is a boroncrosslinked guar designed for delayed crosslinking and optimized for lowguar concentrations. It is made for example with a guar or guar slurry,boric acid, solid or concentrated sodium hydroxide, and sorbitol as astabilizer/delay agent; it may contain clay stabilizers such aspotassium chloride or tetramethylammonium chloride, additionalstabilizers such as sodium thiosulfate (usually obtained as thepentahydrate) and triethanolamine, bactericides, breakers, and breakeraids. A particularly preferred example of this fluid, used for exampleat temperatures below about 110° C. (about 230° F.) is made with about2.16 g/L (18 ppt or pounds per thousand gallons) guar; 2 L/1000 L (2gpt) of a 50% tetra methyl ammonium chloride solution in water; 1 L/1000L (1 gpt) of a non-emulsifying agent containing about 30 to 50% of ablend of alkoxylated polyols, resins, and hydrocarbon solvents inmethanol, propan-2-ol and xylene; 2 L/1000 L (2 gpt) of a surfactantcontaining a mixture of about 15% ethoxylated C₁₁ to C₁₅ linear andbranched alcohols in water, isopropanol and ethylene glycol monobutylether; 0.74 g/L (6.21 ppt) boric acid; 1.74 g/L (14.52 ppt) causticsoda; 2 L/1000 L (2 gpt) of an 85% triethanolamine solution in water;and 2 L/1000 L (2 gpt) of a 48% d-sorbitol (delay agent) solution inwater. The fluid may optionally also contain a breaker such as, but notlimited to, ammonium persulfate or sodium bromate. This formulation isfor example used at a guar concentration of about 1.92 g/L (about 16ppt) to about 3.6 g/L (about 30 ppt) with the amounts of additiveslisted above; preferably for example at concentrations up to about 2.88g/L (about 24 ppt) with 1 to 2 L/1000 L (1 to 2 gpt) of the 50% tetramethyl ammonium chloride solution in water; 0-1 L/1000 L (0-1 gpt) ofthe surfactant described above; 1-2 L/1000 L (1-2 gpt) of thenon-emulsifying agent described above; 0.74 g/L (6.21 ppt) boric acid;1.74 g/L (14.52 ppt) caustic soda; 0-2 L/1000 L (0-2 gpt) of an 85%triethanolamine solution in water; and 1-3 L/1000 L (1-3 gpt) of a 48%d-sorbitol solution in water.

The second preferred metal crosslinked polymer system is a zirconiumcrosslinked carboxymethylhydroxypropyl guar (CMHPG) that is particularlysuitable for example at temperatures between about 79° C. (about 175°F.) to about 121° C. (about 250° F.), in particular above about 110° C.(about 230° F.). This fluid is made for example with about 2.64 g/L(about 22 ppt) carboxymethylhydroxypropyl guar and about 20 g/L (167ppt) KCl; 4 L/1000 L (4 gpt) of a 30% sodium thiosulfate solution inwater (gel stabilizer); 0.1 L/1000 L (0.1 gpt) of a 75% acetic acid and14% propan-2-ol solution in water; and 0.52 L/1000 L (0.52 gpt) of 23%zirconium lactate (crosslinker) solution in a methanol (14%)-watersolution. This formulation is for example used at acarboxymethylhydroxypropyl guar concentration of about 2.64 g/L (about22 ppt) to about 3.6 g/L (about 30 ppt) with for example about 20 to 50g/L (about 167-417 ppt) KCl; about 2-7 L/1000 L (2-7 gpt) of a 30%sodium thiosulfate solution in water; about 0.1 to 0.12 L/1000 L (0.1 to0.12 gpt) of a 75% acetic acid and 14% propan-2-ol solution in water;and 0.45 to 0.65 L/1000 L (0.45 to 0.65 gpt) of 23% zirconium lactatesolution in a methanol (14%)-water solution; preferably at about 3.00g/L (about 25 ppt) with 20 g/L (167 ppt) KCl; 4 L/1000 L (4 gpt) of a30% sodium thiosulfate solution in water; 0.12 L/1000 L (0.12 gpt) of a75% acetic acid and 14% propan-2-ol solution in water; and 0.52 L/1000 L(0.52 gpt) of 23% zirconium lactate solution in a methanol (14%)-watersolution.

The preferred concentration of fiber is 2.40 g/L (20 ppt) for proppantloadings of (0.12 to 0.36 kg/L) (1 to 3 PPA); 3.59 g/L (30 ppt) forproppant loadings of 0.36 to 0.6 kg/L (3 to 5 PPA); and 4.80 g/L (40ppt) for proppant loadings of over 0.6 kg/L (5 PPA). Fiberconcentrations are generally ramped based on proppant concentrations.The preferred polymer crosslinked fluid (for example those describedimmediately above) is shown in Table 1 below:

TABLE 1 Fiber Concentration g/L Temperature 2.40 3.59 7.19 (° C.) GelConcentration g/L <93 2.16 guar 2.16 guar 2.16 guar 104 2.16 guar 2.16guar 2.64 guar 110 2.64 guar 2.88 guar 2.88 guar 115.5 2.64 CMHPG 2.64CMHPG 2.64 CMHPG 121 3.00 CMHPG 3.00 CMHPG 3.00 CMHPG

With these proppant, polymer, and fiber concentrations, the fluidstability is high enough and the proppant settling is slow enough toprovide excellent fracture conductivity.

As with metal cross-linked polymer systems, any viscoelastic surfactantfluid systems (such as cationic, amphoteric, anionic, nonionic, mixed,and zwitterionic viscoelastic surfactant fluid systems, especiallybetaine zwitterionic viscoelastic surfactant fluid systems or amidoamineoxide surfactant fluid systems) may be also used provided that they aretested for compatibility with the fibers and fiber degradation productsof the invention (neither the fibers or their degradation products orthe chemicals in the fluids interfere with the efficacy of one anotheror with fluids that might be encountered during the job, like connatewater or flushes). Non-limiting suitable examples include thosedescribed in U.S. Pat. Nos. 5,551,516; 5,964,295; 5,979,555; 5,979,557;6,140,277; 6,258,859 and 6,509,301, all hereby incorporated byreference.

As has been mentioned, the fiber degradation products are naturalbreakers for viscosifiers, especially for boron-crosslinked polymers andfor VES systems, although this is not a major factor because of the lowpolymer loadings and because VES systems break inherently easily.

Any proppant (gravel) can be used, provided that it is compatible withthe fibers, the formation, the fluid, and the desired results of thetreatment. Such proppants (gravels) can be natural or synthetic(including but not limited to glass beads, ceramic beads, sand, andbauxite), coated, or contain chemicals; more than one can be usedsequentially or in mixtures of different sizes or different materials.The proppant may be resin coated, preferably pre-cured resin coated,provided that the resin and any other chemicals that might be releasedfrom the coating or come in contact with the other chemicals of theInvention are compatible with them. Proppants and gravels in the same ordifferent wells or treatments can be the same material and/or the samesize as one another and the term “proppant” is intended to includegravel in this discussion. In general the proppant used will have anaverage particle size of from about 0.15 mm to about 2.39 mm (about 8 toabout 100 U. S. mesh), more particularly, but not limited to 0.25 to0.43 mm (40/60 mesh), 0.43 to 0.84 mm (20/40 mesh), 0.84 to 1.19 mm(16/20), 0.84 to 1.68 mm (12/20 mesh) and 0.84 to 2.39 mm (8/20 mesh)sized materials. Normally the proppant will be present in the slurry ina concentration of from about 0.12 to about 0.96 kg/L, preferably about0.12 to about 0.72 kg/L (about 1 PPA to about 8 PPA, for example fromabout 0.12 to about 0.54 kg/L 1 to about 6 PPA). (PPA is “poundsproppant added” per gallon of liquid.)

Most commonly the fiber is mixed with a slurry of proppant incrosslinked polymer fluid in the same way and with the same equipment asis used for fibers used for sand control and for prevention of proppantflowback, for example, but not limited to, the method described in U.S.Pat. No. 5,667,012. In fracturing, for proppant transport, suspension,and placement, the fibers are normally used with proppant or gravelladen fluids, not normally with pads, flushes or the like.

Also optionally, the fracturing fluid can contain materials designed tolimit proppant flowback after the fracturing operation is complete byforming a porous pack in the fracture zone. Such materials can be anyknown in the art, such as other fibers, such as glass fibers, availablefrom Schlumberger under the trade name PropNET™ (for example see U.S.Pat. No. 5,501,275). Exemplary proppant flowback inhibitors includefibers or platelets of novoloid or novoloid-type polymers (U.S. Pat. No.5,782,300). Thus the FPV system may contain a second fiber, for examplenon-degradable or degradable only at a higher temperature, presentprimarily to aid in preventing proppant flowback. The FPV system mayalso contain another fiber, such as a polyethylene terephthalate fiber,that is also optimized for assisting in transporting, suspending andplacing proppant, but has a higher degradation temperature and wouldprecipitate calcium and magnesium without preventive measures beingtaken. As has been mentioned, appropriate preventive measures may betaken with other fibers, such as, but not limited to, pumping a pre-padand/or pumping an acid or a chelating dissolver, adsorbing or absorbingan appropriate chelating agent onto or into the fiber, or incorporatingin the fluid precipitation inhibitors or metal scavenger ions thatprevent precipitation.

Although the invention has been described to this point with the use ofmetal crosslinked polymer fluids, viscoelastic surfactant based carrierfluids may also be used with fibers designed to transport, suspend andplace proppant. As with polymers, advantages are that lower viscosities,and thus less chemical can be used, and that the fiber degradationproduct is a breaker. Viscoelastic surfactant based fluid systems areinherently non-damaging.

Any additives normally used in such treatments may be included, againprovided that they are compatible with the other components and thedesired results of the treatment. Such additives can include, but arenot limited to anti-oxidants, crosslinkers, corrosion inhibitors, delayagents, biocides, buffers, fluid loss additives, etc. The wellborestreated can be vertical, deviated or horizontal. They can be completedwith casing and perforations or open hole.

Other forms of fluids sometimes used as carrier fluids or fracturingfluids may also be used in conjunction with the FPV system. For example,the carrier fluid may be an emulsion or may be foamed or energized.

The present invention can be further understood from the followingexamples.

Example 1

FIG. 1 shows the decomposition rate of a suitable fiber of theinvention, a polylactic acid containing about 87 weight % polylactide,about 12 weight % water, and about 1 weight % sizing. The material wasNatureWorks™ PLA 6201D or NatureWorks™ PLA 6202D, made into a fiber ofaverage length about 5.7 to 6.3 mm, and denier about 1.35 to about 1.45.It has been found that the degradation rate is about the same for 6201Dand 6202D. It can be seen that the fiber decomposes in about 1 day at121° C. (about 250° F.) and at about 2 months at 79.4° C. (about 175°F.).

Example 2

The viscosities of a number of polymer-based fluids were determined atvarious temperatures as a function of time at temperature in a Fann 50viscometer. Each fluid contained 3.60 g/L (30 ppt) of the same fibers asused in Example 1. Fluids were made in a Waring blender; in each case,the fluid was made by adding slurried polymer to water, hydrating thepolymer, then adding other additives, then adding fiber to the lineargel before the crosslinking step, and then adding crosslinker. Thoseskilled in the art know how to make metal crosslinked polymer fluids inthe laboratory and in the field, and they know the variations that canbe made (for example pre-mixing certain components, or using batchmixing or continuous mixing methods) to the following general procedure.Specific fluids should be tested in a manner similar to this before use.The runs with fluids 1-7 were done at 93° C. (200° F.); fluid 8 was thesame fluid as fluid 7 but the run was done at 107° C. (225° F.); fluid 9was the same fluid as fluid 7 but the run was done at 121° C. (250° F.).The times shown were the times after the fluids reached the intendedtemperature. The fluids were as follows:

Fluid 1: (A preferred 2.16 g/L (18 ppt) borate crosslinked guar of theinvention.) The fluid contained 2.16 g/L of guar; 2 L/1000 L of a 50%tetra methyl ammonium chloride solution in water; 1 L/1000 L of thenon-emulsifying agent described previously; 2 L/1000 L of the surfactantcontaining a mixture of about 15% ethoxylated C₁₁ to C₁₅ linear andbranched alcohols in water, isopropanol and ethylene glycol monobutylether; 1.13 g/L (9.4 ppt) d-sorbitol; 1.6 L/1000 L of 30% sodiumhydroxide solution in water; and 0.55 g/L (4.6 ppt) boric acid.

Fluid 2: (An unsuitable 2.16 g/L (18 ppt) borate crosslinked guarfluid.) The fluid contained 2.16 g/L (18 ppt) guar polymer; 2 wt % KClor 2 L/1000 L tetramethyl ammonium chloride; 2 L/1000 L of thesurfactant containing a mixture of about 15% ethoxylated C₁₁ to C₁₅linear and branched alcohols in water, isopropanol and ethylene glycolmonobutyl ether; 1.3 L/1000 L sodium tetraborate (borate crosslinker) ina hydrocarbon slurry; 4 L/1000 L sodium thiosulfate; 4 L/1000 L of 85%triethanolamine solution in water; 2 L/1000 L of a 30 wt % solution ofNaOH in water; and 2 L/1000 L of 0.06 kg/L boric acid in water.

Fluid 3: (An unsuitable 3.6 g/L (30 ppt) borate crosslinked guar fluid.)The fluid contained 3.6 g/L (30 ppt) guar polymer; 2 wt % KCl or 2L/1000 L tetramethyl ammonium chloride; 2 L/1000 L of the surfactantcontaining a mixture of about 15% ethoxylated C₁₁ to C₁₅ linear andbranched alcohols in water, isopropanol and ethylene glycol monobutylether; and 3.5 L/1000 L of a crosslinker containing about 10 to 20%sodium tetraborate decahydrate in a mixture of glycerol and water.

Fluid 4: (A suitable 2.64 g/L (22 ppt) borate crosslinked guar of theinvention.) The fluid contained 2.64 g/L (22 ppt) guar polymer; 2 wt %KCl or 2 L/1000 L tetramethyl ammonium chloride; 2 L/1000 L of thesurfactant containing a mixture of about 15% ethoxylated C₁₁ to C₁₅linear and branched alcohols in water, isopropanol and ethylene glycolmonobutyl ether; 2.1 L/1000 L (2.1 gpt) of a crosslinker containingabout 10 to 20% sodium tetraborate decahydrate in a mixture of glyceroland water; and 0.4 L/1000 L (0.4 gpt) of a 30 wt % solution of NaOH.

Fluid 5: (A preferred 2.16 g/L (18 ppt) zirconate crosslinked CMHPGfluid of the invention.) The fluid contained 2.16 g/L of CMHPG; 20 g/Lof potassium chloride; 0.1 L/1000 L (0.1 gpt) of a 75% acetic acid and14% propan-2-ol solution in water; 2 L/1000 L (2 gpt) of a 30% sodiumthiosulfate solution in water; and 0.52 L/1000 L (0.52 gpt) of 23%zirconium lactate solution in a methanol (14%)-water solution

Fluid 6: (A preferred 2.4 g/L (20 ppt) zirconate crosslinked CMHPG fluidof the invention.) The fluid contained 2.4 g/L of CMHPG; 20 g/L ofpotassium chloride; 0.1 L/1000 L (0.1 gpt) of a 75% acetic acid and 14%propan-2-ol solution in water; 2 L/1000 L of a 30% sodium thiosulfatesolution in water; and 0.52 L/1000 L (0.52 gpt) of 23% zirconium lactatesolution in a methanol (14%)-water solution

Fluids 7-9: (A preferred 2.64 g/L (22 ppt) zirconate crosslinked CMHPGfluid of the invention.) The fluid contained 2.64 g/L of CMHPG; 20 g/Lof potassium chloride; 0.1 L/1000 L (0.1 gpt) of a 75% acetic acid and14% propan-2-ol solution in water; 2 L/1000 L (2 gpt) of a 30% sodiumthiosulfate solution in water; and 0.52 L/1000 L (0.52 gpt) of 23%zirconium lactate solution in a methanol (14%)-water solution

The data show the difference between unsuitable, suitable, and preferredfluids. Unsuitable fluids cannot achieve or cannot maintain adequateviscosity. Suitable fluids achieve useful viscosity for a useful periodby starting with high viscosity. The preferred fluids lose viscosityonly very slowly and so do not require high initial viscosities; in factbreakers might be advantageous with such fluids. Those skilled in theart know of suitable breakers (such as but not limited to enzymes oroxidative breakers) and how to use them.

Although the methods and compositions of the invention have beendescribed primarily in terms of stimulation of hydrocarbon producingwells, it is to be understood that the invention may be applied to wellsfor the production of other materials such as water, helium and carbondioxide and that the invention may also be applied to stimulation ofother types of wells such as injection wells, disposal wells, andstorage wells.

The invention may be applied to any type of well, for example cased oropen hole; drilled with an oil-based mud or a water-based mud; vertical,deviated or horizontal; with or without sand control, such as with asand control screen. Other treatments may be performed before or afterthe treatment of the invention, for example scale inhibition, matrixtreatment, killing, lost circulation control, injection of spacers,pushers, pre-flushes, post-flushes, etc. The treatment of the inventionmay be through coiled tubing. In other words, the chemistry,configuration, tools, etc. used in drilling and completion and othertreatments before or after the application of the invention are notcritical, provided that any fluids used or encountered do not interferewith the fluids and materials used in the invention; this may be checkedreadily by simple laboratory or simulation experiments in which thepotential interactions are tested under expected conditions to ensurethat there are no deleterious effects.

Having thus described our invention, we claim:
 1. A method for treatinga subterranean formation, comprising: injecting a fluid comprised offibers and a carrier fluid into the formation; and forming fiberreaction products that do not precipitate in the presence of calcium ormagnesium ions, wherein the carrier fluid comprises a polymer that ispresent in the fluid in a concentration between about 1.92 g/L (about 16ppt) and about 3.6 g/L (about 300 ppt), and wherein the fibers areselected from the group consisting of substituted and unsubstitutedlactide, glycolide, polylactic acid, polyglycolic acid, copolymers ofpolylactic acid and polyglycolic acid, copolymers of glycolic acid withother hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containingmoities, copolymers of lactic acid with other hydroxy-, carboxylicacid-, or hydroxycarboxylic acid-containing moieties, and mixturesthereof.
 2. The method of claim 1, wherein the fluid further comprises apH control agent present in an amount sufficient to neutralize any acidpresent in the fibers before the injection and to neutralize any acidgenerated by the fibers before the fracture closes.
 3. The method ofclaim 2, wherein the pH control agent is selected from the groupconsisting of amines and alkaline earth, ammonium and alkali metal saltsof sesquicarbonates, carbonates, oxalates, hydroxides, oxides,bicarbonates, and organic carboxylates.
 4. The method of claim 2,wherein the pH control agent is selected from the group consisting ofsodium sesquicarbonate, triethanolamine and tetraethylenepentamine. 5.The method of claim 1, wherein the fluid further comprises acrosslinker.
 6. The method of claim 5, wherein the crosslinker comprisesa material selected from the group consisting of boron, titanium,zirconium and aluminum.
 7. The method of claim 1, wherein the fluidfurther comprises a surfactant.
 8. The method of claim 7, wherein thesurfactant comprises a material selected from the group consisting ofbetaine and an amidoamine oxide.
 9. The method of claim 1, wherein thefluid further comprises proppant.
 10. The method of claim 9, wherein aviscosity of the carrier fluid is insufficient to suspend the proppantand a viscosity of the fluid is sufficient to suspend the proppant. 11.The method of claim 1, wherein the injecting the fluid is at a pressureabove the fracture pressure of the formation.
 12. The method of claim 1,wherein the forming fiber reaction products occurs between about 4 hoursand 100 days after the injecting the formed fluid.
 13. The method ofclaim 1, wherein the carrier fluid comprises an emulsion.
 14. The methodof claim 1, wherein the fluid further comprises a crosslinking delayagent.
 15. The method of claim 1, wherein the fibers have a denier ofabout 0.1 to about
 20. 16. The method of claim 1, wherein the fibershave a denier of about 0.15 to about
 6. 17. The method of claim 1,wherein the fibers are coated.
 18. The method of claim 1, wherein thefibers are coated with a hydrophobic material.
 19. The method of claim1, wherein the fluid further comprises a polymer.
 20. The method ofclaim 19, wherein the polymer comprises a material selected from thegroup consisting of guar, hydroxypropyl guar (HPG),carboxymethylhydroxypropyl guar (CMHPG), carboxymethyl guar (CMG),diutan, xanthan, scleroglucan, and hydroxyethylcellulose.